The present invention relates generally to a strain gage balance beam of the type used in wind tunnel evaluation of aerodynamic shapes in which a model of the test shape is mounted with the cantilever mounted balance beam being instrumented so as to measure aerodynamic forces and moments acting upon the model.
In particular, the invention relates to a wind tunnel balance which includes an axial force and a rolling moment measurement section in which the sections are designed to produce negligible mechanical and electrical interaction.
In wind tunnel experimentations it is desirable to have an accurate method of measuring the normal, yaw, and axial forces and the pitching, yawing, and rolling moments on a wind tunnel model through a large load range using only one wind tunnel balance. Prior types of strain gage balances have been successfully utilized to measure the forces in wind tunnel models. The moments and forces acting on the model are usually resolved into the three components of force and three components of moment by providing different members within the balance that are sensitive to only one or two components. Each of the members carry strain gages which are connected in combinations that form Wheatstone bridge circuits. By appropriately connecting the strain gages, the resulting Wheatstone bridge circuit unbalances can be resolved into readings of the six components of force and moment. However, the normal and yaw load interaction on rolling moments (roll) and axial forces (axial) are quite high for a typical balance (approximately 6 percent on roll and 13 percent on axial) so that the accuracy of axial and roll load measurements are affected to a large extent by the magnitude of the normal and yaw loads.
As an example of axial interaction, assume that a 50 lb. normal force and a 5 lb. axial force is statically imposed on a wind tunnel model using a conventional balance. The 50 lb. normal force would produce an apparent load of 6.5 lb (50 lb. .times. 0.13) of axial load with the balance's instrumentation recording 11.5 lb. axial load of which 6.5 lb. would be attributed to the interaction phenomenon. Calibration of the balance can mitigate the effects of interaction to within about .+-. 5 percent which in the above example would be equivalent to .+-. 0.325 lb. However, the effects of this error can be readily seen in that 0.325 lb. is 6.5 percent of the 5 lb. applied and desired to be measured axial force. This accuracy is insufficient to satisfy the needs and demands of the aerodynamist. To attempt to counteract this inaccuracy a balance rated at 50 lb. would have to be used cutting the axial error down to .+-. 3.25 percent which would be barely acceptable.
Roll interaction in the presence of a pitch or yaw load is more severe even though the percentage of interaction is theoretically much lower under these loads. The rate of normal of yaw force to rolling moment can be much larger, for example 50:1 so that the roll interaction for a 50 lb. normal or yaw force would be 3 in. lb. of rolling moment (50 .times. 0.6). The actual applied rolling moment would be 1 in. lb.; therefore, the balance jwould be measuring 4 in. lb. of which 3 in. lb. would be interaction. At the same .+-. 5 percent interaction calibration accuracy, the error would be .+-. 0.15 in. lb. With an actual applied rolling moment of 1 in. lb., the error would be .+-. 15 percent which again is unacceptably too high.
Thus it should be apparent that several balances of varying load measuring sensitivities are needed to cover the full load range of test capabilities desired in any wind tunnel facility.